Deductions & credits

Hello, I have a situation where a client wants to re-fi his pure rental property using his personal residence HELOC loan. That interest is a lot lower.  Under traceability that would let him deduct it on Sched E ( I think)

but the rental is not securing the HELOC.  That makes it a skewed situation  but may be allowable.

I dont see a clear discussion on this in the Code.

I hate to tell him yes , than later on IRS disallows and it won't be able to deduct it on personal Sched A because

he didn't spend it to improve his personal residence.

Thanks in advance for any input here.