That's probably exactly what happened. They had a working relationship that went a little further, he read that as her opening the door for some kind of affair and thought his best course of action was to jerk it over the phone, it freaked her out, from there he went all in on the creepiness with sending gifts and stuff.
I guess she could be lying, but about what really? He admits to doing it... Probably a good lesson in if you want to escalate a relationship (in general, but especially with someone involved with work), be really damn sure they're on the same page and want that extra attention. Also, if you're married, maybe just focus on that relationship.