When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I did not find any weight ratings for our car seats in the repair/maintenance manual. Looking at the sketch below, I suspect the vulnerabilities will be at all the moving parts. I'd suggest to avoid moving the seat while your friend is in it - that means to position the seat to accommodate him/her beforehand.
Lower the seat and back it up to provide comfortable space for him/her. The bottom cushion should be OK since it's anchored on all 4 corners. The back cushion is more vulnerable since it's anchored only on the 2 lower corners - so try to keep it near vertical to avoid excessive load.
I'm not fat by any means, just to clear that up. I workout a lot. I'm 230 plus minus 5 pounds. I daily my Ls for past 2-3 years with no seat issues. But I don't see myself getting anywhere near 300. You can preset the seat setting back and lower, so the motors don't stress moving with him on the seat. And drive slower over pronounced bumps.
I'm pretty much your standard NFL O lineman size standing 6'5 & 315lbs. The LS400 was without issue although over time the springs in the more superior seats did settle some when compared to the passenger. In the LS430 I too find them as solid. However the weak point is the sliding seat cushion in the drivers seat. I have cringed a few times when I heard it lightly 'clicking'. To prevent this I try not to slide it out more than 60-70%.
Take rkw77080's advise & have to seat preset before he arrives.
My son recently had a 400 lb. + passenger in the '01 twice - both front and rear passenger side. He said other than the car leaning a little, and hearing some noises coming from the rear seat when the guy sat down, no known damage done. He drove very slowly (not over 30 mph) and for only about 1+ mile distance each time. rkw's advice is very logical since the front seat is on a scissors for height adjustment, and the seat back is only held upright at the seat base in 2 places. Lower the seat all the way down, and place the seat back in the upright (vertical) position.