I bought a used Sigma (low round count) off a guy.
The trigger is indeed miserable- the best that can be said is that it's consistent; the pull is extremely heavy. However, it's quite reliable, very ergonomic, and if you learn to handle the trigger it can be shot accurately.
IMO, there's really no new pistol that can be had at a similar or lower price that's worth the time. But not much more can put a person into something that is easier to master. I'd definitely take the Sigma over the assorted Taurii, and if that's the gun she wants, then by all means do it. An Apex trigger might be worth it (haven't done it to mine), but I'd probably see if she liked the M&P before plunking down cash on the Sigma.