I'd make sure your lease covers this situation, but then I would also take care of it as a one-time courtesy for the tenant, especially if they pay on time and are good to you.
I'd also consider whether you can do anything to prevent this from happening in the future. For example, in my personal residence's upstairs bathroom, I know I have to remove the drain cover and clean it out with pliers every 3 months. If I had a tenant who called me about the slow-draining tub, I would pay to fix it and then change out the drain style to something less likely to clog, such as something with a grate cover.
I'd also keep in mind that hair going down the drain in a shower or tub is normal. If it's indeed hair plugging the drain, and not a toy/hygiene/diaper item, it's really up to the landlord to improve the configuration to prevent this. Alternatively, if it's something the tenant can fix themselves (like I have to do at my property), you could train them in on the procedure.