Horror movie, female killer, on the beach perhaps?

..Or Psycho Beach Party (even though its made in 2000)

Nope, neither of those.

Ok, first thing- I'm the 'friend' who's looking for the movie- and no, Sudden Impact definitely wasn't it (I actually coincidentally just re-watched that recently, so I can say with 100% certainty that it wasn't. Plus, I remember the movie I'm looking for was a horror movie, and came out before Sudden Impact). Happy Birthday To Me sounds very, very familiar. I'll definitely look into that and report back here. I almost think that might actually be it...