I would think your client would receive a 1099int from the us bank, which should be reported on 1040 schedule B. If you believe it is not subject to us tax due to US-France treaty then back it out on 1040 schedule 1, and fill out the supporting statement. I would put "pursuant to US-France treaty article 11"

Just curious, you say client has been in France for years, what has he done on past returns.