I agree with other's here, losing a steady paying tenant may not be worth the loss you'll take with a vacancy and turn-over costs. I would definitely take the $25 she offered and keep her. Then re-evaluate in a year.
I recently raised a rent on a good tenant by only $10 because she wanted to sign a new yearlong lease. I think that even a small increase sends the message that you're running a business. Seems like in a year, the barista daughter could maybe kick in another $10 or $15 a month?
I understand where you're coming from. I was in a similar situation that I would handle differently if I had to do it over. I kept a tenant's rent artificially low for way too long. As other's have stated, there's a difference between somewhat below market as a sort of "bonus' for a good tenant, and ridiculously below market.
I have one tenant who is substantially below market right now, because a) she's been there almost 20 years, and is low-maintenance, and pays on time, and b) because her unit really needs some substantial updates that I just haven't been able to make happen. When they do, the rent will go up, but as @Marcia Maynard said, a repair/replacement like your oil tank situation really shouldn't factor into an increase, but I do understand how painful that can be when the tenant is enjoying a particularly low rent.
Whenever I send a letter to raise someone's rent, I include the date of their last increase along with a line saying that "due to increased costs of utilities, insurance, and other costs," they should expect a small increase annually. Some years I raise it, some years I don't.
Best of luck!