Just be a supportive presence in your friend's life, no matter what happens, I think they will appreciate it. I know you're concerned about them, but what have they told you about their current situation? How they are feeling about all of this? I think it's important to focus on that.
At the end of the day, taking a medication (or not) is an individual's personal decision and they have to weigh up the risks and benefits for themselves. When you are desperate for relief, often one will try things that carry risk, simply because the existing symptoms are too much to bear, you know? Once someone has access to all the necessary information, only then can they make a truly informed choice.
Typically, prescriptions should come with a safety and information leaflet which goes into full detail about all of the not so pleasant info like potential constraints and side effects, which ideally everyone will read before they start taking a drug. Pharmacists are also a great resource of knowledge if your friend is concerned.
Second generation antipsychotics like risperidone tend to be safer than older ones, typically, if that eases some of your worries. Everyone's experience will be different, of course, but a key mechanism of action of risperidone is an antagonism of multiple types of dopamine receptors, which will always run the risk of extrapyramidal side effects as dopaminergic neurotransmission is vital for motor functioning.
However, lower doses of the drug sway towards antagonizing serotonin receptors rather than dopamine receptors, which lessens the risk of adverse motor-related sequelae. Unless your friend is in active psychosis, it is unlikely they are going to be given the higher doses, so the risk of the particular side effects you're describing would likely be lower, if this quells your worries at all.
What tends to be more worrisome and needs to be monitored carefully are metabolic changes, antipsychotics can cause elevated blood glucose levels and put you at risk for developing diabetes. Serotonin receptors are actually quite important for feeding behaviours, metabolism, weight gain, etc, so it's something to keep an eye out for and make sure your friend informs the prescribing doctor if they're showing signs of being pre-diabetic. Personally, I did not take neuroleptics when I was offered them, for this reason.
All you can do is respect your friend and be there for them no matter what they decide to do. I understand where you're coming from, because I also don't like antipsychotics on principle (that's because I did a neuroscience degree and I have read countless studies about them and how they are used innapropriately in a lot of cases especially in elderly populations). However, if your friend thinks this is the best decision for them and the medication ends up being beneficial for them, I think that is a much much better outcome than ctb.