figure out which motherboard version of slim you have. also, important to know what OFW you have already installed on it.
Recently, i started with a Slim with OFW and installed CFW 5.00M33-6 (not yet on those GEN-X) without using PB and MMS. Hellcat's Recovery Flasher + chickHEN R2 + some OFW + CFW = easy to install. you also got those xgen upgraders as an alternative method.
the conditions are that you have to get to 5.00 OFW installed to get CFW 5.00M33-6. if you have the latest OFW 6.10 or is it 6.20 then you might be stuffed.
i reckon sometimes its better to hold off upgrading to the latest firmware (upgrade/update) - especially when the OFW of PSP has always been a counterstep measure taken by Sony in their battle against evil. Sony dangles some carrot in releasing new system features but then carries a large stick of patching up loopholes and exploits to stop homebrew from running.
also TA-088v3 motherboard (version of Slim) does not run full cfw but it runs homebrew enabler. and then i think, there may be the later mb version of slim which are more troublesome installing cfw, but not sure about my assertion here.
which motherboard version? - check the battery compartment for serial number, date code, the OFW initially installed, the serial number on the retail packaging, etc.
a bit of a catch 22. if you got chickHEN or MHUspeed up and goin, then you would be able to identify your mb by using homebrew PSPident and Dark Alex's Motherboard identifier (not real name, can't remember). but these homebrew identifier s/w are to differentiate betw the TA-088v3 and TA-090v1 (i think) from the rest - to determine whether to install a full CFW or a homebrew enabler. remember you need to have a hacked psp to run homebrew on it.
i am not sure, if you have the undersirable mb, what happens when you try any of these upgrading methods - you would want to know what happens if it bricks and then if it does brick, whether you can recover/repair using the pb and mms.
i have a pb and mms handy, for just in case of an emergency - to repair bricked psp. you can soft- or hard-mod to create a pb but then you could get a 'pre-fabricated' pb.
if your mate purchased the slim a while back then its highly likely to be hackable with full cfw. hopefully, your mate hasn't upgraded the OFW on it to the latest...