I don't think it's rude to give a gift card for a smallish amount - it's the gift that counts, not the price tag.
You know, I am FIRMLY of the opinion that it is the gift that counts, not the price tag. That's why I HATE gift certificates. They negate this rule. The ENTIRETY of that gift IS its price tag. That's the whole point. And if the price tag is not enough to cover a reasonable expenditure at the place in question, then the gift is as good as money tossed down the drain, IMO. He could have given her a certificate at a more affordable restaurant for the same amount and it would have been a much nicer gift b/c it would have been one that she could have enjoyed.
It's rude to give a gift that requires the recipient to spend money they wouldn't have otherwise. That's why I didn't get my brother a satellite radio for Xmas, even though I know he would have loved it. No, the boss is not required to give her a gift at all, and CERTAINLY not required to extend the gift to include the recipient' husband. But it's just common sense etiquette, IMO, that if you want the recipient to enjoy the gift you get them then you make sure you give them something they are in a position to enjoy. For a restaurant GC, that means dinner for 2.
To the OP...I think your friend should tell her boss, "Oh no, we haven't used it yet. We are saving it for a special occasion!" That would keep the boss from asking all the time (hopefully). It's too bad there is no way to clue her boss into the situation so he doesn't do this to everyone he buys a GC for! "Oh we're very excited to try it! We're saving it for a special occasion so we can justify the expense of DH's meal (chuckle)"
*sigh* No, Not polite and I'm not saying she should do it. but it would save countless of this man's recipients this same headache in the future.