It definitely sounds like your wife has a grip problem, and not an ammo problem (but some grip problems can be mitigated with different ammo). Does she shoot the P95 well with other ammo? The UMC might be a little less energetic - or a little less energetic - than other stuff she has shot through the gun. A little more energy can help overcome limp wristing, but then it can also make the gun twist more in her hand - and then cause a loose grip for subsequent shots.
A grip sleeve, skate tape, or bike inner tube grip sleep, can make a world of difference with her grip. I noticed that I couldn't get a high hold without a grip sleeve - after the first shot - because the gun would torque and slip in my hand.