Reyna took a 76th minute free kick after being fouled and from 25 yards out forced a diving save from goalkeeper Florian Kastenmeier.
In the 90th minute, Reyna shot down the middle to Kastenmeier from 20 yards after a layoff from Sebastien Haller, whose return to action after being diagnosed with a testicular tumor six months ago was the focus of BVB's celebrations.
"Sebastien's comeback was amazing," Reyna was quoted on BVB's website. "We're delighted that he's back. He's such a good guy; he works hard. You immediately see his quality on field. He brings something new to our game."
Haller came on for the final 15 minutes.
"We haven't played soccer together in this form for a very long time now," BVB coach Edin Terzic said. "Ultimately, it was very clear-cut; however, not everything was good. But we liked the reaction. We've focused on many areas in the past few days. We saw many of these things today. ...
"It was a very special moment for Seb and his family, but for us too. It was the first time that he was wearing a Borussia Dortmund shirt rather than a training top. We're proud that he has taken this path and that we could experience this today."
BVB, which started its Marbella camp on Jan. 6, is in sixth place, nine points behind leader Bayern Munich and two points below the Champions League qualifying zone. It hosts Augsburg Jan. 22 when the Bundesliga returns from winter break.