HSA contribution not allowed if changed plans mid year

My mother recently retired but in the beginning of year received $2000 from her company for her HSA since she was in a HDHP. She also had additional payroll contributions taken out. She retired in June. The contributions, both employer and employee amount to $5000. Was she only allowed to get deductions for $3375 ($8100 - family plan divided by 12, times 5 months)? She switched to a retiree ppo plan with her company. Would the HSA company be notified that she switched to a non HDHP plan and over contributed, or is the illness on her to report the over payment? Will she have to pay the company back or will the required excess distribution become hers?