First step is to find out what the law is where the property is located. Some places do not require them if the property was built before regulations for the specific area of a building went into effect. In the case of a bathroom, in the US, the National Electrical Code was updated in 1975 to require GFIs. If the property was built before 1975 and NO electrical updates were made since then, you are not required to have them UNLESS local laws require them.
If you are not in violation of the law, tell the tenant that you are not obligated by law to install them, that it should have been brought up prior to signing the lease, but you will be happy to have the work done if tenant agrees to split the cost - adding that you will agree to meet with an electrician of tenant's choice (you should do your own checking for price comparison - if the prices are close, you can be sure that the tenant isn't 'padding' the cost in a deal with the installer, decreasing their share.) It would not be fair to ask tenant to pay the entire cost since this is something you will benefit from after they are no longer living there.
It's not something the tenant can break the lease over unless it's not legal, in that case, get them installed immediately.
If she is a tenant, that is she signed a lease, then she should have given that some thought before she rented. She agreed to the place and you agreed to the price, she wants to change the place you change the price. You do not have to decline it, just have her pay for it if this has become so important for her. Unless GFIs are a building code requirement in your municipality, in which case it would fall on you to be up to code, having her make any enhancement, at her expense while you continue to receive rent and the work is completed during the term of her lease sounds like a good thing. Get it in writing though.
If the outlet is near a water source, in most areas, a GFI outlet is a requirement. I checked the Home Depot website and GFI outlets are as cheap as $12.58--could be cheaper in your area. Since you have 2 bathrooms, you would spend less than $30 for the outlets plus installation (probably less than $50).
It would cost you literally about $5 to convert a normal outlet to GFI. Home Depot or Lowe's sells GFI outlets, you just switch the non GFI for the GFI, takes about 10 minutes each.
Do they even cost that much? How valuable is this tenet? (in terms of longevity, and always paying on time?) Would the GFI possibly save your house from an electrical fire?
To decline, say no, and give her back her deposit (if any) and let her out of her lease without penalty.
the girlie hairdryer will blow the circuit every time........she gonna complain and withhold rent every time...17 bucks parts....300 for an electrician if you can't do it yourself...rental property!....your best friend is youtube
New tenant asks for GFIs installed in 2 bathrooms. We just spent 20k for new floor & sewage before she moved in. How can I decline it? Thanks.