Assuming they are also on Plusnet, or another BT Wholesale ISP - not Sky or TalkTalk, the IP Profile on FTTC is 0.9679 x sync. So either divide IP Profile by that or multiply by 1.033. I always divide, so I only have to remember one number.
On Plusnet there is also the Current line speed
to take into consideration, which is supposed to be the IP Profile rounded down to one decimal place.
On FTTC the sync is set by the DSLAM in the cabinet. IP Profile does not apply until handed over to the BT Wholesale MSAN at the exchange.
I suspect one (or more) of five things is affecting your neighbour's speed.
1 - a different routing to the cabinet, which is possible even with houses next door to each other;
2 - problems between the master socket and the cabinet;
3 - dodgy line card or more likely port in the FTTC cabinet;
4 - dodgy connections in one or both cabinets, most likely the PCP;
5 - dodgy home wiring.
I think the last is the most probable.
My broadband basic info/help site - www.robertos.me.uk.
Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 52.8 (interleaved)/15.0Mbps @ 600m. - IPv4BQM