A good way to say it is

"I know it's none of my business, but it would be better if you could <insert advice here>"

"I used to <insert workout here> like that too, but when the weights got heavy I piled on some injuries that really affected my workout, then I learnt from other lifters that I was doing it wrongly"

"Just passing on some healthy comments, it's up to you to take it. After all, who am I to tell you what to do, it's not like I'm very strong. I just don't want to see a fellow lifter get injured!"