Today is the feast of St. Emerentiana, who I mentioned in one of the poems on St. Agnes' Day. Agnes was a Roman girl from a wealthy Christian family; as was common among the wealthy, she was given to a wetnurse, who was Emerentiana's mother, and who was lactating because she was already nursing Emerentiana. The two girls grew up together, and Emerentiana, who was not from a Christian family, eventually became a catechumen. About this time, according to the story, some of Agnes' pagan suitors who were rejected precisely because they were not Christian reported her to the authorities as a Christian. She was eventually beheaded. A few days later, Emerentiana was found praying outside her tomb near the Via Nomentana by a number of pagans who criticized her for doing so; when she scolded them for the evil they had done to her friend and foster-sister, they stoned the young catechumen to death. She was canonized and is often depicted holding stones in her lap. Both Agnes and Emerentiana are said to be buried in the Roman church of Sant'Agnese fuori le mura (Saint Agnes Outside the Walls), which was built over the catacombs on the Via Nomentana where Agnes was buried and Emerentiana martyred; but it can't be completely ruled out that she is in fact buried somewhere else nearby.
The name 'Emerentiana' is a fairly unusual one (although in some traditions, the Virgin Mary's grandmother is named Emerentia); the name comes from Latin ēmerēre, which means to deserve or merit, especially because you have completed a term of service. It's the same word that gives us the English word 'emeritus' for professors who have completed their service; 'emeritus' was the word Romans used for a veteran who had been honorably discharged.
The building of the church above the catacombs was a common practice in the early Church; it's actually the source of the Catholic customs of dedicating churches to saints, of putting relics in or under altars, and of having patron saints. The earliest Christians would sometimes have their liturgies actually at the graves of martyrs in catacombs and the like. The martyrs themselves were, so to speak, the altars. (In the Emerentiana legend this may well be why the pagans reacted so vehemently to Emerentiana praying at Agnes' tomb.) When it became feasible, they would build churches and basilicas over the catacombs, and of course, the church would be referred to by the relevant martyr, leading to the practice of churches having titular saints. Where it was possible, the churches would be built in such a way that the altar was right over the saint giving the church its title, as Sant'Agnese fuori le mura is built so that the remains of St. Agnes are thought to be directly beneath the altar. But sometimes this was not possible, so it would be done symbolically by putting a relic of the saint in the altar. This continued even as there was a need for churches far from any martyr's grave, and when saints who were not strictly martyrs were given honors analogous to martyrs (thus leading to canonization in our usual sense). Thus, for instance, not far from where I live is a church dedicated to Saint Albert the Great; there is a little bit of bone from St. Albert in the altar, so every time Mass is said, it is in a sense said on the tomb of St. Albert, which in reality is very far from here. Now, these titular saints, especially the more famous ones, were often associated with particular places, which became the idea of patron saints of places, which then expanded, sometimes by accident of location, sometimes by historical association, and sometimes just by analogies based on their lives, to patron saints of guilds and other things. Agnes being the more famous Virgin Martyr, she has a long list of things of which she is taken to be patron saint, most of which are based directly on her life or on historical associations that built up around her titular church. Emerentiana, always the quiet girl in Agnes' shadow, has a much shorter list. Indeed, the only thing I've ever seen Emerentiana listed as patron of is stomach problems. Like most lesser-known saints, the patronage comes purely by an analogy; as I said above, she is usually depicted seated with a pile of stones in her lap, and it's usually thought that she became associated with stomach problems because having a lap full of stones looks like it would be hard on the abdomen. It's far from being the only saint-patronage based on a visual pun.
In any case, I think St. Emerentiana would make a good patron saint for friends.