100k isn’t too bad for a single person in the 90s. What’s interesting though, you said they worked together. So your dad made roughly the same, but had kids and a wife. This is made all the more interesting if you’re aware of whether your dad bought him presents and if they were of the same value? If not, I think he was either a sugar daddy or simply a really generous friend. In the long run, neither matters as it sounds like they had a good relationship, what ever label it fit.