Once a month tops, take it from all the old heads on this subreddit. Back when rolls were the most common form of party drug I knew people who would take em every few days. A couple of em never stopped into their late 20s and you'd swear they were retarded or at least "special" in some way. People who do it regularly for years will fuck up their brains' reward system and deal with emotional problems for years. They don't always show up right away either, they can pop up in your late 30s seemingly out of nowhere.

The reason you have such a bad comedown is because you're borrowing happiness from the next few days so you can be super happy for a few hours now. You do that enough and you'll rewire your brain to the point where it's more like borrowing happiness from the rest of your life so you can be super happy now.

On top of all of this she's probably doing damage to her heart that she won't notice until later in life. Don't get me wrong ecstacy is fun but doing it every weekend isn't only a bad idea but you also won't be getting the full experience if you don't wait a while between trips. You're wasting money and health, and that's just dumb.