If they win it'll take them to 67 which is still within reach of Liverpool, even if they drop 2 points against United, so they'll still have everything to play for against us. Losing our game would effect their goal difference to a degree which would only be to Liverpool's advantage also.
If Arsenal beat Chelsea and we beat Chelsea it's done and dusted for us regardless of what Liverpool do. If Arsenal hammer them by 3 + it may be we only need to hold them to a draw and require only draw against Spurs to edge them on goal difference.
So yeah we should want Arsenal to w