Would his gf get jealous if we did this?

Since it is for your memoirs why not ask her along too. You can have photo's of everyone together, just you and her, just them and just you and him. This is a win/win for you all if your intentions are purely platonic. She may not want to be in the photo's but at least she is there with him it is not done behind her back or with a long drawn out explanation that could be unsettling to her.