Psychology classes have a reputation for being easy and straightforward at most schools, but the MD who issued the warning might have had a different experience.
OP- If your DD is not also taking gen chem and calc, as do most freshman pre-meds, she should be fine even if psych 101 is a tough class at her school. If she takes one or the other or both, perhaps if she is still not sure whether she wants to do a STEM major, she will probably still be fine if she has at least average levels of preparation for the school she chooses to attend and the classes she takes, and especially if the fourth class on her schedule is not something too time-consuming.
It is my feeling that students who begin a major freshman year have a strong advantage as they can schedule other classes around that major, take classes in optimal sequence, and will not be stuck taking too many difficult classes at the same time towards the end of their college career. So, yes, psych101 freshman year would be a good idea for a potential psych major. Department web pages generally have flow charts to advise students.