I own a small apartment block, and I never allow tenants to do any work on my property.
There are many reasons why, but mostly it's because they often neither have the knowledge or skills to do the work properly, and then their "improvements" become my problems.
For example, I use a mildew resistant paint on my bathroom walls to keep them clear of mildew. If a tenant decides he's going to paint the bathroom, and paints over my paint with his non-mildew-resistant paint, then he's unknowingly caused a lot of damage. I now have to paint over his paint to restore the mildew resistance in that bathroom.
I also have a floor cleaning machine that I use to scrub off the top layer of floor finish after a tenant vacates. That top layer will have dirt embedded into it, and I scrub off that dirty surface layer and mop down new floor finish to restore the shiny polished appearance of the floor. If a tenant goes and puts down floor wax on my floors when they move out, they're actually burying dirt under that new wax, and that just makes it harder for me to remove that dirt.
What you should do is simply draw up a list of HOUSE RULES at the time you enter into any tenancy agreements, and one of those rules would be that tenants are not to do any modifications to the property whatsoever, including, but not limited to: painting, installing security systems and replacing light fixtures. Or, specify that no modifications can be done without the landlady's written permission, and that the work has to be done by a tradesman. The problem is that tenants will seldom ask for your permission in advance for fear you'll say no. So, they go ahead and do the work themselves without asking. This way, if they've agreed in advance not to do any modifications, then they'll know they may lose their damage deposit if they make any changes without your permission.