You have shot yourself in the foot so far. I wouldn't look to make an issue as you entered without notice, and I am sure a state would have sympathy toward a tenant in that matter if she pursued it.
I would inform her you will be performing a safety inspection at a given time period and then walk through and see what is going on. It probably won't be there and following that you could perform repairs if you like.
You will need to have some sort of knowledge of the problem to pursue legal action other than " I just went in her apartment for no reason when she wasn't home without
informing her."
Perhaps send a letter to all your tenats reminding them of lease violations sort of like an informational and not accusatory letter. We send stuff like this to tenants all the time, just a monthly newsletter. Perhaps include a section on drugs and how they affect a lease.
You then aren't accusing her of anything, haven't singled her out, and hopefully she will read between the lines and understand that you "know" what's going on.