> What if they were conscious?

Well, they're not.

> If they aren’t but still a lifeform, that makes it perfectly okay?

According to Jains: No. Violence against plants, insects, and possibly even certain microorganisms is considered unethical.

IMO as an irreligious person: Yes. Life is just a particular form of self-sustaining and self-propagating system. Those properties are of little to no moral value.

Are you sure? What about a stand of trees whose consciousness might just run extremely slowly compared to ours?

About as sure as one can be. It's neither logically nor physically impossible, but the claim that trees are conscious is practically unfalsifiable and is not supported by any substantive evidence. It has nothing to do with "fast" or "slow," no matter how you poke or prod or slice or dice a tree, there's nothing that suggests a capacity for consciousness. I would be less surprised if my friend's dog started speaking perfect Chinese with an American accent.

If anyone cares about plants suffering they should go vegan, as many more plants are consumed to raise animals than would be if there was a direct plant intake in humans for the same amount of calories and nutrients. Ditto for land use, water, CO2 emissions, etc. but let's assume our friend cares strictly about reducing suffering short of starving themselves to death.