Unfortunately, prepositions don't translate easily between languages: you need to know how a particular preposition can be used for each verb in the target language. In English, some prepositions can have twenty or more different meanings. Yes, that does sound like a lot of work, but that's the way it is.

toward is perfect because it can mean getting closer to a goal (Oxford dictionary, meaning 1.1).

for is not appropriate: while some of the other meanings seem plausible, its meaning is to the benefit of when used with help. (Oxford dictionary, meaning 3).

the hearing aid was a great help for my mother

Curiously, when used with helpful, it is perfect, for example

bleach is helpful for removing stains

in works nicely because, when used with a verbal noun (reaching is a verbal noun), it means as an integral part of (an activity) (Oxford Dictionary, meaning 8).

Unfortunately, prepositions don't translate easily between languages: you need to know how a particular preposition can be used for each verb in the target language. In English, some prepositions can have twenty or more different meanings. Yes, that does sound like a lot of work, but that's the way it is.

toward is perfect because it can mean getting closer to a goal (Oxford dictionary, meaning 1.1).

for is not appropriate: while some of the other meanings seem plausible, its meaning is to the benefit of when used with help. (Oxford dictionary, meaning 3).

the hearing aid was a great help for my mother

in works nicely because, when used with a verbal noun (reaching is a verbal noun), it means as an integral part of (an activity) (Oxford Dictionary, meaning 8).