okay, options: a) co-workers - he's an english teacher for the secondary (6th-12th) grades at horace mann school for the deaf and hard of hearing; fellow secondary teachers, either same department or whatever subject she'd prefer. possible former FWB situation or just friends with him awkwardly crushing on her when they first met.
b) study abroad fling - they ended up in the same study abroad crew one summer during college (city/country TBD) and voila, whirlwind romance, cliches upon cliches upon cliches, and never to see each other again until her re-entry to boston.
c) friend of the family - possible familial tie to the man his mom's been dating for years. they see each other over holidays and occasionally around town, dynamic can be close or distant, hot or cold, plenty of flexibility here.