the wizard Donny can finish ("the wizard Donny" sounds strange to me. The way my brain reads it, it's like they're two different people.) his portal, he could save his world (So, he has a different world? Like a planet?) and ours. But according to Sandra (Who's she to him?), he's just a 12-year-old boy building a computer to play an online game.
After Donny's former teammates rob him of the book that served as a gateway to Eganesia (Where is this? The place in the video game?), he enlists Sandra's help
of Sandra the warrior to help him retrieve the pieces he needs to build a new portal. (Why did he need the book? Did it help with portals? Why does he want to build a portal?) They take on (Or: "complete" instead of "take on".) many quests to earn the gold needed to finish their project. Once he returns (from where?), he ’ll be able to end can stop the evil sorcerer Matthew’s reign of terror in Eganesia and our world. (Explain his terror more. Also, don't name too many things. Like I'd say "evil sorcerer" over "Matthew".)
But through Sandra’s eyes, (I feel like you've already said something similar before.) she meets Donny after his friends kick him out of their tabletop
role playing game (I feel like it's D&D. If it is, you can just say D&D.). Lacking friends their (Don't you mean "her age" cause we're talking about her here.) age, she helps him retrieve the parts he needs to build a computer (Does she do this so she can make a friend or rather doesn't have friends, so she's bored.). They take on some odd jobs , like mowing lawns—in which she does most of the work , by the way—to earn the money needed to finish their project. She doesn’t care who Matthew is, but if this computer doesn’t work, she’ll have wasted her whole summer. At least, she’ll get a friendship out of it. (The last sentence sounds weird.)
But who’s story right? (Some say to not end a query with a question. I'd think about rephrasing it, but the question works for me. The answer below doesn't work though because you want your readers to think about the question more to see who's right rather than already think "well yeah, there's the end of the story" even if it's not. You don't want them to think they know the end.)
Sandra is. Wizards don’t exist.
Overall, I like the query. The story sounds interesting. I'd just tighten up some words and add in some information.