Should former Catholics still participate in the mass?
No, I don't think they should participate in the mass (that is, have communion). The reason is that its conceptuality is one of the most serious mistakes of the Catholic Church.
I revisited a Catholic mass recently, for a funeral, and it had been a long time since my last visit. When I watched it again I was so appalled that I wanted to walk out. I really wanted to scream, it was so awful. The language that was used about sacrifice, the kissing of the table, the kneeling down—it was all just so offensive to me that I could hardly stand it.
I think that participating in the Catholic mass comes close to compromising the faith, because it is believed to be a reenactment of the sacrifice of Christ and a saving ordinance.
So no, I wouldn't recommend that someone participate.