Multi-dog Parents - Advice Needed

Precursor - coming to the end for my boy.

I have two chocolates, a 13 year old male and his sister an 8 year old female. My boy has had a good run however the Dr. thinks it may be time. My boy has a number of swollen lymph sacs (confirmed not tumors/cancerous), Laryngeal Paralysis (breathing issues due to a weak larynx), and of course the dreaded hip dysplasia. Being a Lab though, he acts like he is still 4 and wants to go, go, go.

My question(s) to other parents, what to do with my girl when it comes time to let him go and last sleep. As the days go on, I have been considering whether or not put him to sleep before he chokes and suffocates in his sleep (which would be beyond scarry for him) or to wait until last minute. Also, what do I do with her? Do I find a way to take her with my when its that time so she can "say goodbye" so to speak and be with him? I really don't want to have to think about this but I know the choice will have to be made or made for me.