Have they been suffering any negative effects? Like depression, anxiety, paranoia, etc? Is it really affecting their lives negatively at all? It takes me a few months of weekend use before it starts getting bad. Every weekend isn't healthy over the medium to long term, but 3 weeks is nothing. You lose most of the good effects if you abuse MDMA too much & it's not very addictive, so it tends to be self limiting. It is possible to develop a dependence on it, but it takes some real dedication, ends up no fun. Finding out the hard way isn't usually very hard with MDMA, as long as they notice any subtle symptoms & stop, rather than just take more.
If you notice they stop having such a great time on it, start using it midweek, or if they get any negative effects - midweek depression is usually the first, that's when to have a word.
Weekly use is almost guaranteed to cause some problems after a while, but it's different for everybody, some people can handle it for months without a problem. Usually they'll find out when they need a break.
Encourage them to get test kits too, so they at least have some idea of what they're taking. You can't stop them taking drugs, even getting them to moderate use could be pretty impossible if they're only getting good effects so far, but you can encourage them to do it in less harmful ways without spoiling the fun & help them to notice any problems their use causes.
