Friends of mine who have done gap years have found it difficult to settle back into uni afterwards, but it's not as difficult as it is made out to be. With the friends thing, I wouldn't worry too much, you'll probably make new friends in the same year group as you anyway. Plus, it's always good to have later year friends who can let you know who the good lecturers are and what subjects to avoid. You won't be an outsider! At uni, it's really normal for people to take leave, gap years, go on exchange, etc.
I wouldn't worry about employability at all. Probably about 30% (estimating here) of uni grads I know graduated later than expected because of any combination of a gap year, going part time or reducing their load per semester, taking leave, changing degree direction, changing degrees altogether... Graduate recruiters are also not concerned about age (that would be discrimination) just that you have completed a degree. Mature age students often apply for the same roles too.