Has she worked or used the NHS while being here?
Can you answer this too as it is important, not only for when you sponsor her for a spouse visa, but for further entry into the UK as a visitor.
Visitors cannot work and have to pay to use the NHS. UKVI (UK visas and immigration) check NHS records (her NHS number) and HMRC records (her NINo). If she has used the NHS, she will need to pay for it and show a receipt for that. Unpaid NHS bills totalling £500 plus, are a refusal of entry or to remain in the UK.
If she is working, then stop. If she has been working for your son's business, then this might affect him too as an employer must check the status of their employees to ensure they have permission to work in the UK. If she is driving, then stop that too.
An overstay alone is not too bad for a spouse visa (but will likely affect when she can apply for British citizenship) but working and using the the NHS without paying (if she has) needs to be sorted. You will need legal advice if she has been working to help/advise/guide with her spouse visa application.