The National institute of Drug abuse categorizes Kratom as a drug with effects similar to opioids.
Marijuana is also a mind altering drug.
To consider casual use of marijuana as a slippery slope assumes the person is in recovery and this is dancing on the edge. Based on the idea that Kratom has effects similar to opiods, one might wonder if active recovery has actually begun.
That said, I don't know if this is a minor, in which case you have a right an a duty to protect the person from illegal or illicit drugs, or an adult who really needs to make decisions for themselves.
And, if there are minors present and other family members are doing illegal or illicit drugs, one might wonder whether or not attending such an event is supporting drug use in general and what type of example this sets.
Trying not to be too judgmental here, but golly - if it walks like a duck...
If you substituted the word "poison" or "cyanide" for marijuana in your question, what would your answer be? A certain amount of cyanide does not kill, but enough does - just like drugs.
Offering Prayers that you gain the clarity you need to make the best decision for you, your addict and your family.