I have no engagement ring and I'm fine with that. We became engaged 31 years ago and had no money. I made it very clear spending money on rent was much more important than a ring. I don't believe in going into debt, especially at that stage, for 'stuff'.

Present the ring to him with no strings attached. Make a suggestion it could be a temporary or permanent engagement ring and leave it to him to present to her. Make it clear the ring is his to present at this time or not.