Depends. Depends, if rate rise quickly, as mentioned here (which IMO will not happen this time), could he pay it off quickly, before it exceeds his current 6.38% amortized 30 years loan? If he could, why not? Personally, I did the same thing, when the prime was 5%, as I had an "investment" that would pay 5.8% (and now the next adjustment will be, for my HELOC either 3.5% or 3% - and yes I could pay it off today if I had to which I would if Prime goes over 6.3%. Ours is Interest Only, 15 year term, and we have withdrawn (used to line) to close to the limit; so freeze it, if they like, it will not impact us. Seems to me the prudent use of a HELOC is good personal financial economics - if you are careful and do not go overboard. Better than most debt as it has the added benefit of the interest being tax deductible.
OP: I just hope your "friend" can act quickly, if and when necessary, or he/she could be in a worse situation than they may be now. Finding an extra $350 a month is good but be sure they look down the line about 5 to 10 years and consider the potential bad as well as the good consequences of what is contemplated.
Vietnam Veteran, CW4 USA, Retired 1979