Ah, apology, was reading it the other round (as departing ZRH on SQ F)

Yes, in his case, arriving on SQ F and departing on LX J, he's down to Business lounge access. And the Alpine lounge, if he can enter Schengen.

SQ Arrivals should be clean (At least LH always arrives clean in Germany when from SIN) so taking the detour via A for the Alpine would save him the security check.

On the other hand, since OP arrives clean in E Gates, he could try (no guarantees!) the FCL in E, especially if his BP has "LX9001 First Class" on it. (Not sure what gets printed in SIN in such case of a codeshare booking) - if that doesn't work, it's a good chance they'll just direct-transfer him from the FCL entrance over to the SEN/C side on the same level, so not much time lost that way. He could always play the "newbie" card And I'd not rule it out that he'll actually be let into the FCL, either due to mistake or due to staff being generous.