Does "to help" only have indirect object without having a direct object?
The charity gave them money [to help them [purchase a house]].
If there's only one object in a clause it is always a direct object.
In this example the matrix clause (the whole sentence) has "money" as direct object and "them" as indirect object. The embedded subordinate clause (in outer brackets) is an adjunct and has "them" as direct object of "help".
The further subordinate clause (in inner brackets) is complement (not object) of "help" and has "a house" as direct object of "purchase".