I feel this would depend on the situation through which they gained their position as a research assistant. As you mentioned "graduate research assistant", I'm assuming this is a position where the masters (or similar) student receives funding for their studies by spending time working in the lab on the advisor's project.
In the UK, a research assistant position could be like this, but would involve a contract of employment. As such, it would be expected that an RA would be present and working on their assigned project, since their pay was coming from the associated grant or project funding.
If that's the case and the student is under contract, then I suspect the situation encountered here is somewhat understandable, if not reasonable.
Depending on the funding for the advisor's research project, they may need to fulfil a certain staffing quota - they have project deadlines and deliverables if it's funded by a grant, and they'll have work needing done to meet these. For that reason I believe it's reasonable for the advisor to want to replace them in the lab. It also might be difficult to find someone suitably experienced, who would be willing to do this for a short period, making it easier to transfer the position to someone else.
To perhaps consider another motivation of the advisor, perhaps they view their RAs as potential PhD candidates - one who is going to move to industry might be less desirable for them. I am merely suggesting this could be a consideration, and I am not suggesting that it would be appropriate.
Regarding question 1, if there is a contract in place, I feel it's reasonable for the advisor to take this position - note they're not preventing the student; simply saying they can't keep the post open indefinitely for them to return to. The funders want results, and replacing the RA is the best way to do that, in my opinion and experience.
Regarding question 2, I think this depends on the student's circumstances - if this will be be beneficial for their career and they can afford to not work as an RA, it seems a good idea to take the opportunity. It's not possible with this information to gauge if there could be any other impact for the student though, so they should consider any potential risks or adverse factors (needing to rely on this advisor as a reference for example).