My bf &his ex ..What do u guys think

So my bf of two years and his ex still keep in contact. They only dated for a few months until she moved away . Now she's moved back in town and her and my bf have hung out 3/4 times I hung with them a few times nothing sketchy. She's actually cool. The only problem I have is them hanging alone . They both smoke weed and drink. The other night she ask him to come over to smoke and he told her he couldn't but could some other time . I told him I feel uncomfortable with them hanging alone& "getting high" and he respects it . Sometimes I feel guilty for requesting that cause she is cool but me and him had problems awhile back with him sexting a chick friend while he was drunk and I think that situation alone is asking for trouble. I am building trust and I know I have no control over someone's actions . Am I being reasonable tho?