"We are now past 4/15 - can the client simply withdraw the excess given the extension?"

No. The deadline is 4/15.

"He was advised by the brokerage bank to file form 5330"

Yes. You need to determine if that is the "employee" share or the "employer" share.

I found some articles for your reference:

https://mysolo401k.net/mycommunity/forums/discussion/rules-for-correcting-solo-401k-over-contributio...

https://www.mysolo401k.net/dealing-excess-nondeductible-solo-401k-individual-k-contributions/