I think you mean another manga, because I remember there's one that has similar plotline, a part timer falling for his boss and it ends up like you described. The boss is divorced though (or lived together with his girlfriend until she suddenly disappeared). Since he worked there every summer for years, he knows it all.
Oh that might be it! Do you happen to remember the title or author?
Finally find it after a long search (⌒▽⌒)
http://www.mangago.zone/read-manga/umi_to_futari_no_enbun_noudo/
When Shunsuke is already a working adult and helps Aki with the shop's accounting. I just can't for the life of me remember where I've read it!