A Thai people who is issued a Schengen Visa category D 'long stay visa' ( she's mother of EU kid )

can travel with just a one way-ticket ? or some "stupid" airlines will again a required a return/ownard ticket ?

I know that with a short-stay "C" visa it's need ( return/ownard )

A type "D" visa is an entry visa for immigration, long term stay. A single entry will be sufficient for the autorities and airliners should be aware too. If you wish to make sure your airliner is competent, you could double check with them. The sadly do sometimes lack common sense or general visa knowledge: it's not their task to decide about entry into the country but they are scared for being fined if the traveler is denied entry.

A return ticket is also not mandatory for type C (short stay) visa. Though the traveller must convince the authorities that they will return in time so in most cases this is with a (reservation of) a return flight. But other options are possible: heading on to other countries outside Schengen, funds to buy a return ticket later. To avoid haasle, in by far most cases a return ticket is the most convinient for C type visas.