would you be more comfortable with two lines attached? Toss the main halyard shackle end forward over the spreader and attach both the main and foresail halyard to the chair. Use two people to hoist him the odds of both failing are slim. Also have him climb on the way up and down don't try to pull his full weight up the mast. Just an idea, I do it with my wire rope splices and I'm ok with it. I can't speak for the condition of his lines though.
"who za sheet is kingsley zissou"...claus R\V Bellefonte