A client is asking me to fill a W9 with my name and the LLC EIN instead of a W8 which seems to be wrong. What’s the worst that can happen if he sends the incorrect form to then IRS and who has to amend the potential issue? Can I be held liable in any way or is my client responsible for asking me and submitting the wrong form?
Neither W-9 or W-8 is filed with IRS; quid's comment is correct on that point.
However, since you are a non-US person (aka NRA, nonresident alien) and the payments to your disregarded (single-member) LLC are treated as payments to you -- see pub 515 -- depending on the type of income the payer may be required to withhold using the information that they should get on W-8, and liable if they fail to do so. If they instead treat you as a US payee, which is what W-9 implies, they aren't required to withhold but may be required to file reports on the payments using the W-9 info, and e.g. if this payment is for work, IRS will detect that their 1099-NEC does not match to a resident return on 1040 (or 1040SR!) with schedule C from you. Whether IRS will do anything about it, especially if you file 1040NR and pay as required, is a different question; I'd expect this is low on their priority list. And they'd probably go after the payer first, both because the payer is in US jurisdiction whereas you are not, and because if a business is paying you wrong there's a good chance they're doing so for other people too, whereas there is no indication you are being improperly paid by other US payers.
See the (extensive) links at https://www.irs.gov/individuals/international-taxpayers/nra-withholding