A lady i took riding lessons from said her purebred arabian mare got covered by a shetland.ruined bloodline?

she said it happened some yrs back. the mare had a pure bloodline,n the stallion they had already paid to have her covered by was pure arabian. however, the day before they were to trailer her over to the pasture he was in, her little shetland stallion "got wind" of her,and broke from his pasture,and covered her. the foal that resulted,she said, ruined her mare's bloodline. is this true? does one mongrel pony a destroyed bloodline make?