In general a fighter may still use any card to boost, as long as it has a boost value, no matter who the card is for or who is/isn’t on the board. However, only one of Sinbad’s Voyage cards (The Voyage Home) has a boost value. Only card with an actual printed boost value may be discarded to boost. You can only voluntarily discard those boostless cards when you have more than 7 cards in hand.
Edit: oops, I think you’re asking about Sinbad’s team ability and I answered something slightly different. I’ll just leave it anyway in case other people have the same confusion looking for the info.
Edit: oops, I think you’re asking about Sinbad’s team ability and I answered something slightly different. I’ll just leave it anyway in case other people have the same confusion looking for the info.