Thanks a lot for all your help!
My friend doesn't think she has any physiological problem preventing her from equalising, since it immediately happens when she swallows. But since she says she needs something like 30 seconds to gather enough spit to be able to swallow, if she descends at the rate of one swallowing per meter, by the time she's reached the desired depth, she'll almost have to ascend again.
On top of that, she was scared she'd harm her eardrums by the force she had to apply when trying to do the Valsava manoeuvre. Since no matter how hard she tried, she only succeeded in equalising once using Valsava during alll her OWD dives, it looks like the VM has more risks than advantages for her, doesn't it?
In spite of all, she didn't have any ear problems after the dives (other than, as I said, her ears popping every time she swallowed for many hours afterwards). She just says that whenever she finally managed to swallow, the pop was incredibly loud (which I suppose proves pressure behind her eardrums had built up a lot, though not enough to cause any harm).
Gurning might be a solution. I'll let her know about it. If not, is there any way she could produce more spit underwater? Anxiety won't help, for sure, but the solution a well-meaning fellow OWD student suggested, chewing gum, sounds too crazy even to consider it.