Roth Conversion Rules involving Non-Deductible IRA

I have what I think is somewhat complicated. I have a client who has the following:

• $66K in a Non-Deductible IRA (About half is cost basis and the other half is growth)
• $250K in an “old” 401K she would like to rollover into her IRA

Question is, if she does this and later wants to convert the non-deductible portion of her IRA into a Roth is this further complicated by having done the 401K rollover? I thought I read somewhere about pro rata rules pertaining to Roth Conversions where a portion of your overall Traditional IRA account(s) are non-deductible vs. deductible.

If this is the case, does it make sense to first convert the non-deductible (Traditional IRA) then do the Rollover. I welcome any guidance that can provided here. Thanks in advance.