Is there any source saying that Buddhists can temporarily form relationship to help people? There are many good answers in the question Would Buddhists help non-Buddhists continuing their attachments?. I'll summarize some of the points here:


Buddhism only teaches that things don't last, not stop doing them. Love people knowing that someday we lose them; use things knowing they break; earn money to survive not out of greed; eat food only to be healthy not because taste good. Even if the girl is attaching to her mother, helping them or not will not make them less attaching anyhow. (ashen25's answer)
Dhammadhatu's answer even cites the suttas: 


  Buddhism has two levels of teaching: (i) moral, which includes attachment; and (ii) non-attachment (MN 117). The Buddha said his teaching of non-attachment was only for a minority of people (MN 26).
  
  Therefore, a Buddhist would help ordinary people maintain their important social relationships. In fact, this is a duty of a monk (DN 31). The duty of a monk &amp; of a Buddhist is not to "strip" ordinary people of their attachments &amp; identities. 



The consensus in there is clear: helping them, not converting them. However, psychologically speaking, I think helping people necessarily requires both parties forming a relationship, or requires the helpers involves/attaches in other people's emotional dramas. Or when they are helping them, they need to have expectations on the outcomes. These are things a Buddhist would like to avoid.

They can simply say "sorry, my goal is not to get attached with relationships. I'm not a suitable person to help you. I hope you get well with your life." Not only this is an acceptable manner, this would also conveniently help the helpees understand the value of non-attachment without the need to teach them anything.

I think the solution is simple: as long as the helpers acknowledge that after the problem has been solved they can detach to it, then it would be fine. However, are there any sources discussing this?
The following three quotes from the question are merely more annihilationist wrong views (per SN 12.17) based on the original annihilationist wrong view:


Requires the helpers involves/attaches in other people's emotional dramas. 
When they are helping them, they need to have expectations on the outcomes. 
My goal is not to get attached with relationships. I'm not a suitable person to help you. 


The above ideas are "annihilationist" (per SN 12.17) because these views ignorantly assume a "self" exists that must non-attach from itself &amp; from other "selves", "persons" or "beings". 

When the mind is enlightened with right view, it does not think it is helping "people". It is only helping "ignorance" become wiser. Since the enlightened mind has no view of "persons" or "beings", it naturally also has no attachment when helping. 

Please refer to SN 12.17 and also MN 102. 


  Just as a dog, tied by a leash to a post or stake, keeps running around and circling around that very post or stake; in the same way,
  these venerable contemplative &amp; brahmans—through fear of
  self-identity, through disgust for self-identity—(nevertheless) keep
  running &amp; circling around self-identity. 
  
  MN 102


Therefore, if &amp; when "helping necessarily requires both parties forming a relationship", one party (the helped) is attached &amp; the other party (the helper) is non-attached. In other words, the helper is free from fear &amp; is unconcerned about whether the other party attaches to them. The helper trusts in virtue, appropriate boundaries &amp; the Dhamma. The helper does not need to and should not physical touch the helped, let alone engage in sexual relationship. Thus, pure metta will always be harmless to both, even if the helped becomes attached. When a Virtuous One acts with pure metta, the helped will always remember &amp; love them but this type of love is not harmful. 

As Volkov wisely said elsewhere:


  As my teacher explained, it's only when the person is completely
  abandoned by the currents of normal life, is when they get picked up
  by the current of Dharma.


In other words, those looking for help can even spend months in a monastery, where they may speak each day for one or two hours with a monk. But, eventually, such ordinary worldly individuals will leave the monastery when they feel better or restored (to try their luck again in the world of attached relationships).