It really is impossible to say because even without a cold sore you can be infectious. All you can do is be safe and wait until the area is completely healed, so no redness at all. Also you might want to mention to her so you can both make an informed decision, better to be safe than sorry.
Okay, so my cold sore is at that point where its starting to curl up where its snagging on fabrics and stuff, so its close to falling off, if there is still any reddness or anything is it very likely she will get infected? even if there is no spot or signs is there a high chance of infection? or is it rare?
You are least contagious when there is no cold sore present at all, so wait untill the red scab is gone. It's still possible to infect someone with the virus even if a cold sore isn't present.
^^Im sorry i didnt mean to post that! x{