I think what he's saying is that the clarification in RMC SL states that you cannot directly affect the unwilling with a U spell, and that if you use trickery to make them willing, there are instances where treating the U as an F and doing the BAR/RR thing is appropriate.
He would prefer to be allowed to use U spells offensively, especially for dramatic contrast of target and spell level. (i.e. a 30th level U effect can be ignored by a 1st level target merely by being unwilling per the rules.)
My suggestion, is that if you are going to allow some or all U spells to be treated that way, to merely make U spells into F spells, but apply a penalty, as you have.
Do realise that certain low level U spells are sickly powerful if treated as F spells, and set the bonus accordingly. (It takes very little heat/cool solid/liquid to kill a person, and you can usually do it at less than 5th level.)
Your players should be made aware of this fact, and that the goose/gander logic means it can and will happen to them. (i.e. if they can boil someone's blood, then no whining when it happens to them.)
This isn't a game breaking modification, I've played in games where U are treated as F, no modifier at all, just straight BAR/RR effects. . .it had two noticeable effects on gameplay:
1) Shifted power signifigantly in favor of casters vs non casters.
2) Increased the number of spell failures, since BAR is more likely than BSC to produce them.
That's a high end "High magic game" house rule. . .you can moderate it with RR bonuses vs U, and/or by sticking with the text of the spells as written. (i.e. "Heat liquid" is not only a "U" spell, it also specifically states that it only works on inanimate liquids.)