The trainer's first heal will always be used if the base criteria is met. 

Base criteria for HP healing items: Pokemon has HP < 25%
Base criteria for for Full Heals: Pokemon has any status condition

For subsequent heals, if # of mons remaining > # of heals remaining + 1, then the heal won't be used. Otherwise it will be used if the base criteria is met. The number of heals remaining appears to include both HP healing items and Full Heals.