Agreed! It's the next best step he can do before filing. He should go there in person to discuss with them exactly what he is doing and ask for their help.
What's most important is to make sure you have properly completed all paperwork before you file and make sure you provide the beneficiary with any other additional evidence before the interview. That includes: photos, receipts, divorce certificates, birth certificates, copies of the original package and so on. They should have a large bag to carry it. It most likely won't be needed, but better to have too much than not enough.
It's long been debated as to whether the petitioner should attend the interview, and many think as the OP stated that's it's good to be there. Truth is, due to work, money, or logistics it's not often possible for the petitioner to attend. If they are able to be with their partner for the interview, they will not be allowed to accompany them. They must wait until the interview is over. The USCIS assures all petitioners and beneficiaries that attendance is NOT required or has any bearing whatsoever on how the case will be adjudicated. So, for those who may be worried that their case may be rejected because they are not able to attend the interview, do NOT worry. You will be fine; especially if you make sure you provide and prepare the beneficiary for any contingency.
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