Well I would make contact. 15 years is a long time. When my youngest DD was about 3 I made friends with another mum also with a 3-year old (boy). We soon had a group of mums (and the odd dad!) who got together regularly and once at school we had a rota to pick up one or two kiddies to give their mums a break. In later years we holidayed together (our DHs got on well and later still we formed a quiz team at our local pub. We often had "dinner parties" together and I would have considered her one of my closed friends. Her DS invited our DD and us to his wedding and later our DD did the same in reverse.
Then it suddenly went wrong and I don't know why. There was a spell of about two months when I wasn't able to get to the weekly quizzes (I worked 45 miles away) and on one occasion, intending to go, I suffered a TIA and was rushed to hospital. My other friends told me later that she was furious that I had let them down with no notice!!
After that she simply didn't keep in touch (she lived about 200 yards from me) and the other ladies in the group said they had no idea why. It is all about 15 years ago now.
Last week I heard that she is suffering from Dementia. I was most upset as she had been such a lively woman. She is probably about 78 and her DH is at least 6 years older.
I haven't got in touch yet, but I will do. She may or may not remember me, but her DH will and I just think it is neighbourly to offer my support in whatever way I can, if only emotional support to him. We were friends for almost a lifetime.
So I would get in touch, see how the land lies, and accept that if it doesn't work out you at least offered the olive branch.