It's illegal for him to operate IFR period.

Really? Even if you don't fly in the clouds?

If he is in a case where that reg applies to him, yes. The rules don't
say "in clouds" they say Instrument Flight Rules.

I think that all the icing forecasts I've seen say ICGICIP (icing in clouds
and in precipitation), so if you stay out of the clouds and precip there's no
forecast icing.

Barry