The Terps overcame a two-set deficit to send the game into a pivotal fifth set.
Haley Ellefson knocked in the winning run to give the Wildcats their second loss of the season in game one.
Jared Bernhardt scored five goals to break Matt Rambo's all-time scoring record.
Costes knocked the go-ahead, two-run single in the sixth inning as the Terps picked up their third straight win.
The loss concluded a winless regular season for the Terps.
Russell averaged 14.7 points, 4.5 rebounds and 4.5 assists with Rhode Island this season.
Wahab fills a desperate need for the Terps in the low post.
The Terps tallied a 195.075 team score, their fourth-lowest score of the season.
Led by Erika Pritchard's 16 kills, the Terps rallied for their fifth win of the season.
The Terps lost their third straight game by at least eight runs.