No, she can’t do this legally without a visa https://www.gov.uk/check-uk-visa/y ‘Skilled worker’ doesn’t mean “having qualifications but working in unskilled employment”. She can use the tool on the uk.gov site to find out which visa, if any, would allow her to work.
Note that working while in the UK on a Standard Visitor visa is not allowed.
The reason for this rejection seems to be your work experience in company is just 2 months. With in 2 months company sending you for training seems little fishy. As per my experience consular officers prefer or trust atleast the 2 years of current job.