Thank you very much for your reply.
Actually, after reviewing my note, the IRS did an SFR for 2013, 2014, and 2016. I did not get any IRS notice from my client for 2013; I asked him about it and still waiting for his response.
If he, in the end, cannot pay then should I file his 2013 or 2014 first? (The IRS audit exam unit was looking at his 2013 return.)
Or, it does not matter of which year I should file first, but the key is to file 2013 to 2018 before the deadlines.
Thanks in advance for your advice! I asked for $1,000 retainer, but if I have known he owes $280K, I would have asked for more.