Why not just use the same rules you would if your mate had shown up and said 'There is a new guy in work who'd like to join our game' you must have some criteria for letting new people join in? I play woth a group of lads (and a lass or two) from work and we've never said no to anyone joining in, most people realise in the first week if it is for them based on skill levels, fitness etc. Some come back some don't, you said yourself it's only a friendly kick about with your mates and she is no more likely to pick up a serious injury than anyone else it's not like her bones are made of glass!