Leighton Baines has the most assists for a defender in PL history with 50 assists. Can`t find any info about a single season
That so did not answer the question :p
Apparently it is Baines with 9 in the 2014/15 season.
Can't confirm it but can't imagine there's been a better season from a fullback in that regard; taking set pieces helps.
I looked at each season on the PL website.
17/18 - 7 Cresswell.
16/17 - 5 Walker/Trippier/Smith
15/16 - 5 Bellerin/Daniels
14/15 - 9 Baines
13/14 - 7 Kolarov
12/13 - 5 Baines/Coleman/Evra
11/12 - 6 Cole
10/11 - 11 Baines
09/10 - 8 Baines
08/09 - 6 Baines
07/08 - 8 Shorey
06/07 - 5 McCulloch/Neil/Shorey
05/06 - 5 Asier Del Horno/Aaron Hughes
04/05 - 7 Riise
03/04 - 5 Sun Jihai/Kennedy/Naismith
02/03 - 6 Silvestre
01/02 - 9 Mark Venus
00/01 - 6 Harte/Telfer
99/00 - 6 Matteo/Ziegler
98/99 - 7 Ian Harte
97/98 - 8 Gary Kelly
96/97 - 9 Andy Hinchcliffe!
95/96 - 8 Dan Petrescu
-- 22 teams --
94/95 - 11 Andy Hinchcliffe
93/94 - 8 Le Saux
92/93 - 8 Steve Staunton
Mendy could beat that by Christmas if we play 352 enough.