84 300SD restarting woes after filter change UGH!

[DJNEWK2]

Hey guys!

I'm right next to a new (to me) 84 300SD that I'm thinking of purchasing and the previous owner let me take her on the road all day and upon our ride I noticed that she was a bit sluggish on hills and suggested we try to replace the fuel filters to start with so we purchased them earlier. Upon MY return from my own errands he has replaced both filters and of course NOW it won't start. He used STARTING FLUID to try to crank it and it ran until it ran that out of course and then he kept doing it but she won't start. I've told him to just screw the Starting fluid at this point and will just pull the filter and fill with fuel or check the lines individually cause I'm thinking its air... MY question is (since I don't use SF but WD40 if I was to spray anything) since not even SF is working,is that also due to possible air or would it just not burn at all?

What I mean is I'm HOPING it's just air since it ran fine (except for the sluggishness) and now it just turns over whether he shoots SF or not. Any suggestions or opinions for why it wouldn't start even to burn off the SF or would it just not start regardless (because its not reacting to the SF either although she turns over forever)? Just filled the tank with fuel too ugh. Any help is greatly appreciated and thanks in advance fellas!