If she lost her key, she should have to pay for you to get a copy of your spare key cut for her, not you. If you have a spare key, don't give it to her. You don't want her to be the only person with a key to the flat. If there is no spare key and a locksmith has to go out, arrange it yourself so that both of you are there when the lock is replaced, and you control who gets a key. Tell her she has to pay for it, but good luck getting her to do so. If she is not paying rent she'll probably tell you where to go.
Don't play hardball with the key. It is tempting to try to use the situation to your advantage, but it's not worth it. The last thing you want is for her to break a door or window to get into the flat and then report you to the PRTB for preventing her from getting into "her" home. Replacing the former, or paying the fines imposed by the latter would be a lot more expensive than replacing a key.