Do you have a relationship with your friend's girlfriend? If so, ask her straight out if she minds.

If the gf seems as though she may be afraid or angry that you might go with him, try to explain your reasoning. You could explain to her how you can help your friend, using your personal experience (or knowledge of) depression as well as the changes that you have seen within your friend. Hopefully, that will make her comfortable with your offer and she will see the positives of you becoming involved.

I hope that it works out well for your friend and that his gf is okay with your possible involvement, should her bf need your support. It would be great for your friend to have your support ~ someone that he knows personally, to help him through dark times. Best wishes!