Your husband has lost so much of what he was able to do. If he is able to continue, even with some assistance or minor changes, it would definitely be a boost to his self esteem if he could continue this teaching.... especially if the participants are receptive to his teaching. Yes, the time is probably coming when he can not continue but if it is ok for now I would let it be.
Later you could find someone to co-teach with him. Leaving him with just part of the lesson to teach. Rather than just stopping the activity, give him assistance and let him do what he can. As his speech become more difficult he will probably give more and more of the actually teaching over to his "assistance" but still consider himself involved.
We truly need to focus on what they CAN do and try to modify what they enjoyed to fit their new level of cognition when possible. As your informants said so wisely... we all have difficulties with something